This invention relates to open celled polyurethane foams having very low resistance to air flow and to a process for producing these foams.
Flexible polyurethane foams and processes for their production are known and described in the art. In general, these foams are prepared by reacting organic and/or modified organic polyisocyanates with one or more compounds having a higher functionality and containing at least two hydrogen atoms that are capable of reacting with the NCO groups of the polyisocyanates, and optionally, chain extenders and/or crosslinking agents, in the presence of catalysts, blowing agents, additives, etc. The preparation of flexible polyurethane foams is described in Kunststoff-Handbuch, Volume VIII, Polyurethane, 1st Edition 1966, Edited by Dr. R. Vieweg and Dr. A. Hochtlen, and 2nd Edition, 1983, and 3rd Edition, 1993, each edited by Dr. G. Oertel (Carl Hanser Verlag, Munich).
Flexible polyurethane foam is used widely in the comfort cushion market (furniture, bedding, automotive); in the textile area (apparel, blankets); in the industrial packaging and vibration insulating fields; in other household furnishings and sponges; filters, and the like. The versatility of polyurethane foam, permitting its use in diverse markets, results in substantial part from the nature and variety of the raw materials which are used to produce the foam products, as well as the manner in which the raw materials and the resultant foam are processed. Foams ranging widely in density and hardness, in tensile and tear properties, in resistance to compression set and fatigue, in resilience, deformation, recovery rate and hysteresis, in durability and toughness are obtained by selection and variation in raw materials and processing conditions. An important further characteristic in foam that likewise varies widely is its breathability, or resistance to air flow, of the basic cellular structure.
Cellular solid polymer foam has a skeletal framework of relatively heavy strands which form an outline for the cell structure. The strands of the skeletal framework are conventionally connected by very thin membranes, or windows, which form the walls of the cells. In open-celled foams, some of the windows are open or torn in each cell, thus forming an interconnecting network open to air flow. As produced, flexible polyurethane foams contain some cell windows that are either closed or only partially open and are not sufficiently porous or open-celled to exhibit very low resistance to air flow. For certain types of flexible foams such as HR and some viscoelastic foams a mechanical crushing step is employed to enhance air flow; however, this is of limited use for producing very high air flow foams due to windows that are not crushed open and to the residual fractured cell membranes that are still present.
Reticulated foams in which the cell windows have been completely or almost completely removed have a wide variety of applications, including safety fuel tanks, printer rollers, filters, etc. The production of reticulated foams has generally been accomplished by post-forming methods to increase the degree of openess of the cell structure, by breaking or removing the residual cell windows of these foams. Chemical, mechanical shock and thermal reticulation means have all been used.
Reticulated foams could be suitable for other application areas, such as mattresses, pillows, furniture, etc., if they could be easily produced in situ with readily available raw materials and without the need for a separate chemical or mechanical process to remove the cell windows. If these foams could also be prepared in a way such that water penetration and flow through the foams was attainable, then these could also be used in other application areas such as sponges, lawn furniture, gutter guards, wipes, etc.
Various references disclose open celled foams and processes for the production of these foams. These include, for example, U.S. Pat. Nos. 3,433,752, 3,454,504, 4,656,196, 4,670,477, 6,391,933, 6,391,935 and 6,638,986.
Although foams are mainly prepared from higher molecular weight compounds having at least two groups which are reactive with NCO groups of the polyisocyanate, and lower molecular weight chain extenders and crosslinking agents, the use of monofunctional compounds in foam is also known and described. See, for example, U.S. Pat. Nos. 3,405,077, 3,875,086, 4,209,593, 4,950,695, 4,981,880, 5,631,319, 6,136,879 and 6,391,935. One reason to include monofunctional compounds is to produce a softer foam with lower load bearing as in U.S. Pat. Nos. 3,405,077, 3,875,086 and 4,981,880. U.S. Pat. No. 4,950,695 discloses the use of a monofunctional alcohol or polyether to prepare soft flexible polyurethane foams. Formulations in this reference also contain a 2000 to 6500 molecular weight triol. Other references such as U.S. Pat. No. 5,631,319 disclose the use of a C1-C25 monoalcohol in combination with a hydroxyketone to form non-viscoelastic foam. U.S. Pat. No. 4,209,593 describes energy-absorbing foams that are prepared from a naphthol or other bulky monohydroxy compound The viscoelastic foams prepared in U.S. Pat. No. 6,136,879 may contain a polyether monol which has a molecular weight of less than 1500 with a polyol having a molecular weight greater than 1800. All of the examples were prepared at isocyanate indices of less than 90.
U.S. Pat. No. 6,391,935 also describes viscoelastic foams. These foams are prepared from an isocyanate-reactive component that contains a low equivalent weight polyol and from about 15 to 70 wt. % of a polyester or polyoxyalkylene monol having a number average equivalent weight greater than about 1000. Foams of the '935 patent can be prepared over a broad range of processing conditions and isocyanate indices.
Advantages of the present invention include a process for the direct production of polyether flexible foams exhibiting very low resistance to air flow. High air flow is usually a desirable performance trait for flexible foam because it relates to improved recovery characteristics and durability. It has also been associated with improved comfort by enhancing the transfer of heat and moisture away from a body contacting an article manufactured with the foam. These foams also should be suitable for many applications that currently utilize very high air flow foams that can only be produced by subjecting the cured foam to a separate reticulation process. Reticulation is typically a slow and time consuming process that involves the use of specialized and expensive equipment. A direct process for producing very high air flow foams could significantly increase productions volumes and reduce conversion costs opening new high volume uses for the foam. This would be an advance welcomed by urethane foam producers, fabricators and end-users.